Diese Präsentation wurde erfolgreich gemeldet.
Die SlideShare-Präsentation wird heruntergeladen. ×
Anzeige
Anzeige
Anzeige
Anzeige
Anzeige
Anzeige
Anzeige
Anzeige
Anzeige
Anzeige
Anzeige
Anzeige
Nächste SlideShare
HVDC TRANSMISSION LINE  PPT
HVDC TRANSMISSION LINE PPT
Wird geladen in …3
×

Hier ansehen

1 von 124 Anzeige

Hvdc dad

Herunterladen, um offline zu lesen

Introduction, equipment required for HVDC systems, Comparison of AC and DC Transmission, Limitations of HVDC transmission lines, reliability of HVDC systems, comparison of HVDC link with EHVAC link, HVDC system configuration and components, fundamental equations in HVDC system, HVDC links, converter theory and performance equation, valve characteristic, converter circuits, converter transformer testing, multi bridge converters, abnormal operation of HVDC system, control of HVDC system, harmonics and filters. Influence of AC system strength on AC/DC system interaction, response to AC and DC system faults, Concept of reactive power compensation- reactive Power balance in HVDC substations-Effect of angle of advance and extinction angle on reactive power requirement of converters.

Introduction, equipment required for HVDC systems, Comparison of AC and DC Transmission, Limitations of HVDC transmission lines, reliability of HVDC systems, comparison of HVDC link with EHVAC link, HVDC system configuration and components, fundamental equations in HVDC system, HVDC links, converter theory and performance equation, valve characteristic, converter circuits, converter transformer testing, multi bridge converters, abnormal operation of HVDC system, control of HVDC system, harmonics and filters. Influence of AC system strength on AC/DC system interaction, response to AC and DC system faults, Concept of reactive power compensation- reactive Power balance in HVDC substations-Effect of angle of advance and extinction angle on reactive power requirement of converters.

Anzeige
Anzeige

Weitere Verwandte Inhalte

Diashows für Sie (20)

Ähnlich wie Hvdc dad (20)

Anzeige

Aktuellste (20)

Hvdc dad

  1. 1. UNIT 2: HVDC TRANSMISSION M&V Patel Department of Electrical Engineering Faculty of Technology and Engineering Charotar University of Science and Technology – Changa Prepared by: Dharmesh A Dabhi Assistant Professor
  2. 2. HVDC Transmission Introduction, Comparison of AC and DC Transmission, Limitations of HVDC transmission lines, Comparison of HVDC link with EHVAC link ,Equipment required for HVDC systems, reliability of HVDC systems, , HVDC system configuration and components, fundamental equations in HVDC system, HVDC links, converter theory and performance equation, valve characteristic, converter circuits, converter transformer testing, multi bridge converters, abnormal operation of HVDC system, control of HVDC system, harmonics and filters. Influence of AC system strength on AC/DC system interaction, response to AC and DC system faults, Concept of reactive power compensation- reactive Power balance in HVDC substations-Effect of angle of advance and extinction angle on reactive power requirement of converters.
  3. 3. Evolution of power system Late 1870 Commercial use of electricity start. 1882 First electric power system by Thomas Edison at pearl street station(Generator, Cable,Fuse, Load(LAMP)). DC system,59 customers,1.5 km in radius.110v load, underground cable. 1886 Limitation of dc arise. Two aspect (i)High losses and (ii) Voltage drop To minimize two aspect transformers and ac distribution(150 lamps) developed by William Stanky of Westinghouse Electric Corporation. 1888 N.Tesla developed poly-phase systems and had patents of Generator, Motors, Transformers, Transmission lines.(Westinghouse Electric Corporation brought it) 1889 First ac transmission system in USA between Willamette falls and Portland 1-phase, 4000V over 21 km. 1890 Controversy on whether industry should standardize ac or dc. Finally ac use because of some advantages. Voltage increase, Simpler & Cheaper generators and motors. 1893 First 3-phase line, 2300V, 12km in California
  4. 4. EARLY VOLTAGE 1922-165kv 1965-500kv 1923-220kv 1966-735kv 1935-287kv 1969-765kv 1953-330kv 1990-1100kv It is due to various advancement in insulating material. EARLYER FREQUENCY 25,50,125,60,133 Hz USA 60 HZ,OTHER COUNTRY 50 HZ
  5. 5. HVDC trasmission A system of HVDC transmission was designed by French engineer Rene Thury. 1880-1911 At least 11 Thury system were installed in Europe. -DC series generators used and its operating in constant current control mode 1938 All Thury system were dismantled due to safety and maintenance problem. 1950 Mercury arc valve developed so it was possible to convert ac to dc. 1954 First HVDC transmission between Sweden and Gotland island developed. It was 70 km long.ac was not suitable therefore go to HVDC.
  6. 6. • LIMITATION OF HVAC TRANSMISSION 1. Reactive Power loss. 2. Stability. 3. Less Current carrying capacity. 4. Skin effect and Ferranti effect. 5. Power flow control is not possible. • ADVANTAGES OF HVDC TRANSMISSION  No reactive power loss.  No stability problem.  No charging current.  No Skin and Ferranti effect.  Power control is possible.  Cheaper for long distance.  Asynchronous operation is possible.  No transmission of short circuit power.  Require less space compared to ac for same voltage rating and size.  Ground can be used as return conductor.  Less corona loss & radio interference.  No compensation is required.  Fast fault clearing time.
  7. 7. Why HVDC?  DC is more efficient than AC for transmitting large amounts of power over long distances  3,500 MW transmission line, 600 mi. long  Losses on AC line ~ 15%  Losses on DC line ~ 4.54%  Only 2 conductors vs. 9 (3 per phase)  Smaller towers require less right of way  Break even distances  600 km (373 mi.) for above ground lines  50 km (31 mi.) for submarine lines  Allows two unsynchronized AC grids to be connected http://www.electricaleasy.com/2016/02/hvdc-vs-hvac.html
  8. 8. • DISADVANTAGES OF HVDC TRANSMISSION  Cost of the terminal equipment is high.  Introduction of Harmonics.  Blocking of reactive power.  Point-to-Point transmission.  Limited overload capacity.  Huge reactive power at the converter terminals. • Objectives of HVDC transmission; 1. Bulk Power transmission.(Large Power over long distance) 2. Back to Back HVDC.(Control of power and voltage) 3. Modulation of AC.(Stability)
  9. 9. Monopolar link:- • Having One conductor(-ve polarity) and ground is used as return path. • The return path is provided by ground or water. • Use of this system is mainly due to cost considerations. • A metallic return may be used where earth resistivity is too low. • This configuration type is the first step towards a bipolar link.
  10. 10. Bipolar link • There are two conductors. • One operates at +ve polarity and other is –ve polarity. • During fault in one pole it works as Monopolar. • Each terminal has two converters of equal rated voltage, connected in series on the DC side. • The junctions between the converters is grounded. • If one pole is isolated due to fault, the other pole can operate with ground and carry half the rated load (or more using overload capabilities of its converter line).
  11. 11. Homopolar link: • Two or more conductors having the same polarity. • Normally negative polarities are used. • Ground is always used as returns path. • During fault in one pole it works as Monopolar.
  12. 12. Principle parts of HVDC transmission:
  13. 13. CONVERTERS • Converters are the main part of HVDC system. • Each HVDC lines has atleast two converters, one at each end. • Sending end converter works as Rectifier (It converts AC power to DC power). However converter as receiving end works as Inverter ( it converts DC power to AC power). • In case for reversal of operation, Rectifier can be used as inverter or vice versa. So generally it is call it as CONVERTERS. • Several thyristors are connector in series and/or in parallel to form a valve to achieve higher voltage / current ratings. Note*- Valves (Combinations of several thyristors) .
  14. 14. Various Thyristor Ratings:
  15. 15. Thyristor Valves  Module  Contains 6-10 high power thyristors,  High power thyristors  1-10 kV  1-5 kA  Cooling  Constant circulation of deionized water  Isolation  Thyristors operating a high potential relative to ground the gate signal needs to be isolated http://www.industrial-electronics.com/elec_pwr_3e_22.html
  16. 16. Continues… • How to achieve required voltage and current ratings? The current rating of converter stations can be increased by putting  Valves in parallel  Thyristors in parallel  Bridges in parallel  Some combinations of above. The voltage ratings of converter stations can be increased by putting  Valves in series  Bridges in series  Combination of above. Bridge converters are normally used in HVDC systems.
  17. 17. Main requirement of the Valves are: • To allow current flow with low voltage drop across it during the conduction phase and to offer high resistance for non conducting phase. • To withstand high peak inverse voltage during non conducting phase. • To allow reasonably short commutation angle during inverter operation. • Smooth control of conducting and non conducting phases.
  18. 18. Continues… • Two versions of switching converters are feasible depending on whether DC storage device utilized is. • An inductor-Current source converter • A Capacitor-Voltage source converter. • CSC is preferable for HVDC system • VSC is preferable for FACTS like STATCOM,SVC,etc
  19. 19. Comparison of CSC and VSC: CSC VSC Inductor is used in DC side Capacitor is used in DC side Constant current Constant voltage Higher losses More efficient Fast accurate control Slow control Larger and more expensive Smaller and less expensive More fault tolerant and more reliable Less fault tolerant and less reliable Simpler control Complex control
  20. 20. CONVERTER TRANSFORMERS: • For six pulse converter, a conventional three phase or three single phase transformer is used. • However for 12 pulse configuration, following transformer are used.  Six single -phase two windings  Three single- phase three windings  Two three- phase two windings
  21. 21. Continues… • As leakage flux of a converter transformer contains very high harmonic contents, it produces greater eddy current loss and hot spot in the transformer tank. • In case of 12-Pulse configuration, if two three phase transformers are used, one will have star-star connection, and another will have star delta connection to give phase shift of 30°. • Since fault current due to fault across valve is predominantly controlled by transformer impedance, the leakage impedance of converter transformer is higher than the conventional transformer. • On-line tap changing is used to control the voltage and reactive power demand.
  22. 22. SMOOTHING REACTORS: • As its name, these reactors are used for smoothing the direct current output in the DC line. • It also limits the rate of rise of the fault current in the case of DC line short circuit. • Normally Partial or total air cored magnetically shielded reactor are used. • Disc coil type windings are used and braced to withstand the short circuit current. • The saturation inductance should not be too low.
  23. 23. Harmonic filters • Harmonics generated by converters are of the order of np±1in AC side and np is the DC side. Where p is number of pulses and n is integer. • Filter are used to provide low impedance path to the ground for the harmonics current. • They are connected to the converter terminals so that harmonics should not enter to AC system. • However, it is not possible to protect all harmonics from entering into AC system. • Magnitudes of some harmonics are high and filters are used for them only. • These filters are used to provide some reactive power compensation at the terminals.
  24. 24. Overhead lines: • As monopolar transmission scheme is most economical and the first consideration is to use ground as return path for DC current. • But use of ground as conductor is not permitted for longer use and a bipolar arrangement is used with equal and opposite current in both poles. • In case of failure in any poles, ground is used as a return path temporarily. • The basic principle of design of DC overhead lines is almost same as AC lines design such as configurations,towers,insulators etc. • The number of insulators and clearances are determined based on DC voltage. • The choice of conductors depends mainly on corona and field effect considerations.
  25. 25. Economics of power transmission: • The cost of transmission line includes the investment and operational costs. Investment cost includes,  Right of way  Transmission towers  Conductors  Insulators  Terminal equipment Operational costs includes  It mainly due to cost of losses
  26. 26. Right of Way(RoW): • An electric transmission line right-of-way (ROW) is a strip of land used by Electrical utilities to construct, operate, maintain and repair the transmission line facilities. • Rights of way may also include the purchase of rights to remove danger trees. A danger tree is a tree outside the right of way but with the potential to do damage to equipment within the right of way. If the danger tree falls or is cut down, it could strike poles, towers, wires, lines, appliances or other equipment and disrupt the flow of electricity to our customers.
  27. 27. Images for (RoW)
  28. 28. Continues…
  29. 29. Continues… • This Implies that for a given power level, DC lines requires less RoW, Simpler , and cheaper towers and reduced conductors and insulator costs. • The power losses are also reduced with DC as there are only two conductors are used. • No skin effect with DC is also beneficial in reducing power loss marginally. • The dielectric losses in case of power cables is also very less for DC transmission. • The corona effects tends to less significant on DC conductors than for AC and this leads to choice of economic size of conductors with DC transmission.
  30. 30. Continues… • The other factors that influence the line cost are the cost of compensation and terminal equipment. • In dc lines do not require compensation but the terminal equipment costs are increased due to the presence of converters and filters.
  31. 31. Reactive power source. • As such converter does not consume reactive power but due to phase displacement of current drawn by converter and the voltage in AC system, reactive power is drawn. Reactive power requirement at a converter station is about 50-60% of real power transfer, which is supplied by filters, capacitors and synchronous condensers. • Synchronous condensers are not only supplying the reactive power but also provide AC voltage for natural commutation of the inverter.
  32. 32. Earth electrodes: • The earth resistivity of at upper layer is higher (~4000 ohm-m) and electrodes cannot be kept directly on the earth surface. • The electrode are buried into the earth where the resistivity is around (3-10 ohm-m) to reduce transient over voltages during line faults and gives low DC electric potential and potential gradient at the surface of the earth. • The location of earth electrode is also important due to  Possible interference of DC current ripple to power lines, communication systems of telephone and railway signals,etc.  Metallic corrosion of pipes, cable sheaths ,etc.  Public safety.  The electrode must have low resistance (Less than 0.1 ohm) and buried upto 500 meters into the earth.
  33. 33. AC
  34. 34. DC
  35. 35. DC
  36. 36. HVDC BIPOLAR LINKS IN INDIA NER ER SR NR NER ER SR NR RIHAND-DELHI -- 2*750 MW CHANDRAPUR-PADGE – 2* 750 MW TALCHER-KOLAR – 2*1000 MW ER TO SR SILERU-BARASORE - 100 MW EXPERIMENTAL PROJECT ER –SR
  37. 37. HVDC IN INDIA Bipolar HVDC LINK CONNECTING REGION CAPACITY (MW) LINE LENGTH Rihand – Dadri North-North 1500 815 Chandrapur - Padghe West - West 1500 752 Talcher – Kolar East – South 2500 1367
  38. 38. ASYNCHRONOUS LINKS IN INDIA NER ER SR NR NER ER SR NR VINDYACHAL (N-W) – 2*250 MW CHANDRAPUR (W-S)– 2*500 MW VIZAG (E-S) - 2*500 MW SASARAM (E-N) - 1*500 MW
  39. 39. HVDC IN INDIA Back-to-Back HVDC LINK CONNECTING REGION CAPACITY (MW) Vindyachal North – West 2 x 250 Chandrapur West – South 2 x 500 Vizag – I East – South 500 Sasaram East – North 500 Vizag – II East – South 500
  40. 40. Three Phase Full wave bridge converter
  41. 41. Line or phase Current
  42. 42. Analysis including commutation overlap • Due to the inductance Lc of ac source, the phase currents cannot change instantly. Therefore, the transfer of current from one phase to another requires finite time, called the commutation or overlap time. • It is denoted by µ. Typically the full load values of µ are between 15˚ to 25˚. • During commutation overlap, three valves conduct at the same time • Other wise only two valves conduct for period (60˚-μ) with no firing angle delay.
  43. 43. Analysis including commutation overlap Effect of commutation over lap on periods of conduction of valve
  44. 44. Effect of commutation overlap with ignition delay The commutation begins at ωt = α and ends at ωt = α+μ=δ, where δ = extinction angle
  45. 45. Equivalent circuit diagram during commutation
  46. 46. Voltage waveforms showing the effect of overlap from valve 1 to valve 3
  47. 47. Converter angle definition
  48. 48. Power flow in HVDC links • The HVDC system has two converters, either of them can be made to work as rectifier/inverter. Thus the power flow can be bidirectional. • The power flow can be controlled by controlling the DC voltage on rectifier side or inverter side. • Thus the current flowing through the DC link can be given by • Where Vd1 = DC voltage on rectifier end • Vd2= DC voltage on inverter end • R = resistance of the DC line per pole • The DC voltage in the middle of the line is given by • Thus the DC power through the line is given by RVVI ddd /21  RVVV ddd /213  dddc IVP 
  49. 49. Control of Voltages Vd1 and Vd2 • There are two ways to control the value of DC Vd1 and Vd2  With the use of tap changer control which is slow and may take 8 to 10 seconds.  Fast variation in the DC voltages can be achieved through gate control , by varying the firing angle for a few milliseconds.  The value of Pdc can be controlled quickly because of small value of R.  The DC side values of converter and inverter depend on the position of the tap changer connected to their respective transformers.  The general practice is to hold inverter side voltage Vd2 constant at certain value and control the current by rectifier terminal.  The current through the rectifier and inverter is maintained same.
  50. 50. Power at rectifier and inverter end • Power at the rectifier end (Pd1) is given by, = • The power thus can be controlled by controlling the difference of the DC voltage at rectifier and inverter end. • Power at inverter end is given by, = = ddd IVP 11  ddd IVP 22  The negative sign of the first term indicates that the power is received by inverter
  51. 51. Power loss in DC system • The power loss in DC system is • Line loss increases with increase in power flow. • Power loss in middle of line Pdm = (Pd1+Pd2)/2 • = • = RIP dL *2)^(
  52. 52. Abnormal operation: Arc-back• Arc-back: This fault refers to the reverse conduction of the valves. This can happen only when inverse voltage appears across the valves. In rectifier inverse voltage appears in 2/3rd cycle. So arc back are more common phenomenon in rectifier. • The effect of Arc back is to place a short circuit between two phases, due to the reverse current flow from one valve. • To avoid this a bypass valve is placed in parallel with rectifier to provide a diversion for the fault current.
  53. 53. Abnormal operation: Commutation failure • This abnormal condition is more common in inverter. Commutation failure means, failure to turn-off a valve, before the commutation voltage across it reverses. • If valve 1 has to turn OFF and valve three is to be turnd ON, and the valve 1 fails to turn off then after 60 degree valve 4 will be triggered, thus it will place a short circuit across the phase.
  54. 54. Thus from the equations above it can be said that the DC voltage and current at any point in the line can be controlled by controlling the α for rectifier and γ for inverter. Initially for rapid action firing angle control is used and than followed by to tap changing transformer to restore the converter quantities α & γ to their normal range.
  55. 55. Constant voltage mode compensates for the RI drop and thus its characteristic shows constant V. This mode maintains γ to be around18 degree. Thus this mode is less prone to commutation failure. The β control maintains constant β, so the voltage increases slightly with increase in current. In this mode at lower load value of μ is less, but at higher load it increases thus decreasing the value of γ, so it can lead to commutation failure.
  56. 56. Maximum current limit: The maximum current limit is limited to 1.2 to 1.3 times the normal full load current to avoid damage to the valve. Minimum current limit: If the current is discontinuous, then in case of a 12 pulse converter , it will stop and start 12 times. This will lead to high Ldi/dt voltages in transformer windings and due to lower overlap there will be high stress on the switch
  57. 57. Steady state VI characteristic for inverter and rectifier
  58. 58. Control implementation • The control scheme is divided into four levels: • Bridge converter control unit: Defines the αmin and γmin, Fastest response • Pole control: Controls the bridges in a pole; current order to firing angle order; tap changer control and protection sequences such as starting up, deblocking and balancing bridge controls • Master control: current order to all the poles with co- ordination; interface between the pole control and overall control • Overall control: Power flow scheduling; Ac system stabilization and communication between both the terminals
  59. 59. Hierarchy of different levels of HVDC system
  60. 60. Converter firing control systems • Converter firing system decide the firing insants for different control modes: CC,CIA,CEA • Two types of control are used:  Individual pulse control: Firing pulses are decided individually for each valve depending on the zero crossing of their commutation voltage • Advantage: achieve highest possible direct voltage under unsymmetrical or distorted supply waveforms since firing angle is determined independently • Disadvantage: Any deviation of the AC system voltage from its ideal waveform will lead to asymmetry in current waveforms; introduces non- characteristic harmonics  Equidistant pulse control: Valves are ignited at equal time intervals, the ignition angles are retarded or advanced equally • Advantage: Results in lower level of uncharacteristic harmonics and stable control with week AC systems • Disadvantage: If AC network asymmetry is large it results in lower power and DC voltage
  61. 61. Starting, Stopping and power flow reversal Normal stopping (blocking) sequence: The following steps are taken:
  62. 62. Starting, Stopping and power flow reversal • Normal Starting sequence:
  63. 63. • Power flow reversal: • Power reversal can be done in around 20 to 30 ms. Starting, Stopping and power flow reversal
  64. 64. Controls of enhancement of AC system performance The following are the major reasons for using supplementary control:

×